The present invention relates to a loop back control system in a data way system having duplex loop transmission lines.
FIG. 1 shows an example of a conventional data way system having duplex loop transmission lines. Referring to FIG. 1, a line monitoring station (to be referred to as a master station hereinafter) 1 and data send stations (to be referred to as remote stations hereinafter) 2.sub.1, 2.sub.2, . . . , and 2.sub.n are looped through lines LA and LB. The master station 1 performs polling of the remote stations 2.sub.1, 2.sub.2, . . . , and 2.sub.n (i.e., in the order of the remote stations closer to the master station 1) in units of the remote stations. Devices (DIVs) 3 such as computers or terminal equipment are connected to the master station 1 and the remote stations 2.sub.1, 2.sub.2, . . . , and 2.sub.n, respectively.
In a duplex loop transmission system of this type, system reliability can be improved, and expansion and modification of the system can be easily performed by a loop back function forming a return path at any station.
When cable trouble occurs as shown in FIG. 2, or the remote station 2.sub.2 breaks down as shown in FIG. 3, the loop back function is reconstructed under the control of the master station 1. In this case, the system is reconstructed such that the fault section is eliminated, as shown in FIG. 4.
Conventionally, in order to reconstruct the path of the data shown in FIG. 4, the master station 1 performs a message exchange with the remote stations 2.sub.1, 2.sub.2, . . . , and 2.sub.n beginning with the remote stations closer to the master station 1, in the units of the remote stations in accordance with polling. The master station 1 independently sends loop back commands to the remote stations 2.sub.1 and 2.sub.2, to perform loop back control, as shown in FIG. 4.
However, according to the conventional data reconstruction means of the type described above, line asynchronism occurs due to a time-lag between the commands sent to the remote stations 2.sub.1 and 2.sub.n, respectively. In addition to this disadvantage, system reconstruction takes a long time, so high-speed reconstruction cannot be performed, resulting in inconvenience.